different conditions, have an immense advantage in height, weight, constitutional vigor and fertility over the self-fertilizing offspring from either one of the same parents". He thought that this observation was amply sufficient to account for outcrossing sexual reproduction. The disadvantages of self-fertilized offspring (inbreeding depression) are now thought to be largely due to the homozygous expression of deleterious recessive mutations; and the fitness advantages of some outcrossed offspring are thought to be largely due to the heterozygous masking of such deleterious mutations except when such mutations lead to outbreeding depression. See also Consanguinity Heterosis Outbreeding depression References Breeding Dog breeding Plant sexuality